Hello Colm,
I hope you are well.
Looking at your network diagram:
this customer wants to use eBGP multihoming.
the two routers are connected to different ISPs as I see different AS numbers.
First of all, I would use an IGP among the two C6509 and the two 2821.
I would use OSPF because it is possible to use a route-map to conditionate the generation of a default route.
in normal conditions RA generates a default route of type O E1 with a lower metric so that both 6500 agree on using it as exit point.
RA
router ospf 10
! or other network used between the 4
network 2.153.83.0 0.0.0.255 area 0
! point to point with 6509-A if used
network x.x.x.y 0.0.0.0 area 0
default-information originate type 1 route-map check-bgp
prefix-list only-default permit 0.0.0.0/0
access-list 11 permit host primary-ebgp-peer-address
route-map check-bgp permit 10
! check the presence of a BGP default
match ip address prefix only-default
! check default route BGP next-hop
match ip next-hop 11
set metric 50
on RB everything similar but with a bigger seed metric like 100 so that to compensate for OSPF cost of links between the two 6500.
I've used this design successfully and it is effective: if the eBGP 0.0.0.0/0 disappears the router RA stops to send 0.0.0.0/0 in OSPF.
the two C6500 can use HSRP with object tracking to track ebgp peer reachability on C6500-A.
About BGP:
being two different ISPs you can try to use AS path prepending on the secondary as a way to influence return path.
Other possible approach if your customer owns a /23 prefix:
advertise a /24 (or two /24) out primary link
advertise a single /23 out secondary link
this can be combined with AS path prepending.
most specific route is used.
if the whole block is a /24 this approach is not viable most ISPs filter prefixes more specific then /24.
Hope to help
Giuseppe
